Early Modern English diphthongs also underwent a series of changes which masked their earlier sound. Almost all of the changes that took place were the result of processes of monophthongisation: this means that a diphthong was reduced to a pure long vowel.
There were seven diphthong phonemes in late Middle English, namely: iʊ, eʊ, aʊ, ai, ɔʊ, oi and ʊi. (Cf. Barber 1976: 299)
Only two ME diphthongs remained the same and retained their quality in the southern dialects, namely [ɔʊ] as in joy and [ʊi] as in join. Both diphthongs are nowadays often realised as [ɔi].
Two middle English diphthongs, namely [iu] and [eu], became [ju:] in Early Modern English as in view and few. Most of the [iu] words like due acquired the pronunciation [ju:]. (Cf. Nevalianen 2006, 123) In some positions it developed to [u:] after [tʃ], [dʒ] and [r] as in chew or June. In other positions the [ju:] became [u:] since the beginning of the 18th century. In Present Day English one often finds alternating pronunciations in words like lute or suit. (Cf. Barber 1976: 300) The [eu] diphthongs first moved to the position of [ɜu], and then became [iu]. In a third stage it became [ju:] as well. (Cf. Nevalainen 2006: 123)
In the early 18th century the initial [j] in [ju:] began to be lost and the sound was reduced to [u:] in many words, e.g. brew, chew, crew. (Cf. Nevalainen 2006: 123)
The change from [ju:] to [u:] has not really been completed yet as there are many varying in regional speech. (Cf. Nevalainen 2006: 123)
In Early Modern English the two diphthongs ending in an [ʊ] glide, [aʊ] and [ɔʊ], became monophthongs. They shifted their position and became the low back vowel [ɑ:] and the high- mid back vowel [o:]. The Late Middle English [aʊ] in words like all or cause retracted and monophthongised in the first half of the 17th century and resulted in a low [ɑ:]. Later it was raised to a low- mid [əɔ:]. (Cf. Nevalainen 2006: 123) The high-mid back vowel [o:] (bowl, flow) developed out of the diphthong [ɔʊ] that first monophthongised into [ɔ:] and then finally raised to [o:]. It therefore joined in the development of the ME long [ɔ:] in words like boat and home (Cf. Nevalainen 2006: 123) which the great vowel shift had raised to [o:]. The GVS had raised these vowels to [o:] in about 1600 and in the south they became centring diphthongs: [əʊ]. (Cf. Nevalainen 2006: 124)
The [ai] sound that occurred in words like day and bait was first raised to [ɜi] in certain Early Modern varieties, which formed the basis for the merging of the monophthong [ɜ:], later [e:]. (Cf. Nevalainen 2006: 124) This sound coalesced with the raising of the Middle English sound [a:] to [ɜ:] due to the GVS. (Cf. Nevalainen 2006: 124) As a consequence many word pairs with identical pronunciation were created. Examples would be: days/daze, bait/bate, hail/hale etc. In Present Day English these words are pronounced with a [ei] sound. In Late Modern English [ɜ:] words also underwent a process of diphthongisation. This can be seen in spelling differences that show their earlier pronunciation, e.g. [ai] or [a:]. (Cf. Nevalainen 2006: 124)
The Middle English [oi] was [ɔi] in words like noise and royal, and remained until today. However, there are also some words pronounced [oi] that derived from ME [ui]. This diphthong developed into [əi] and changed to [ai] in the Early Modern period. It occurred in words like boil, destroy, join etc. In PDE it is pronounced [ɔi]. (Cf. Barber 1976: 304)
|[ɪʊ:]||[iu]→ [ju:]||[ju:]||new, hue|
|[eu]||ɜu]→ [iu]→ [ju:]||[ju:]||dew, few|
|[aʊ]||[ɑu] → [ɒu]||[ɔ:]||cause, law|
|[ɔʊ:]||[ɔ:] → [o:]||[əʊ:]||soul, know|
|[ai]||[ɜi]→ [ɜ:]→ [e:]||[eɪ]||day, night|
(Barber 1976: 305)